Polar codes, proposed by E. Arikan, “Channel Polarization: A Method for Constructing Capacity-Achieving Codes for Symmetric Binary-Input Memoryless Channels,” IEEE Transactions on Information Theory, Vol. 55, Issue 7, pages 3051-3073, July 2009, are the first class of constructive coding schemes that are provable to achieve symmetric capacity of binary-input discrete memoryless channels under a low-complexity Successive Cancellation (SC) decoder. However, the finite-length performance of polar codes under SC is not competitive over other modern channel coding schemes, such as Low-Density Parity-Check (LDPC) codes and turbo codes. Later, a SC List (SCL) decoder was proposed in I. Tal et al., “List Decoding of Polar Codes,” 2011 IEEE International Symposium on Information Theory Proceedings, pages 1-5, Jul. 31-Aug. 5, 2011. The proposed SCL decoder can approach the performance of an optimal Maximum-Likelihood (ML) decoder. By concatenating a simple Cyclic Redundancy Check (CRC) coding, it was shown that the performance of a concatenated polar code is better than the performance of well-optimized LDPC and turbo codes. This result represents the breakthrough of polar codes in future wireless communications systems (i.e., 5th Generation (5G)).
Wireless broadband systems require flexible and adaptive transmission techniques since they operate in the presence of time-varying channels. For such systems, Hybrid Automatic Repeat Request (HARQ) based on Incremental Redundancy (HARQ-IR) schemes are often used, where parity bits are sent in an incremental fashion depending on the quality of the time-varying channel. IR systems require the use of rate-compatible punctured codes. According to the rate requirement, an appropriate number of parity bits are sent by the transmitter. Here, the set of parity bits of a higher rate code should be a subset of the set of parity bits of a lower rate code. Therefore, in a HARQ-IR system, if the receiver fails to decode at a particular rate, the receiver only needs to request additional parity bits from the transmitter. For this reason, there has been extensive research on the construction of rate-compatible turbo codes and LDPC codes.
Although polar codes can achieve the capacity of a binary input symmetric output channel, existing polar codes are not rate-compatible and, as such, are not suitable for use in a future wireless communications system, particularly one that utilizes HARQ-IR or some similar IR transmission scheme.